The present invention relates to an improvement in the processing of oilseeds, for example, soybean, cottonseed, corn, peanut, safflower, sunflower, and palm. More particularly, this invention relates to improvement in a process utilizing an isopropanol-based solvent to extract oil from such seeds.
The oilseed industry of the United States produces on an annual basis about thirteen million tons of seedoils from roughly one billion bushels of seed crops, predominantly soybean and cottonseed. Essentially all of this oil is recovered from the seeds by solvent extraction. The oils find primary use in foods, e.g., shortening, margarine, cooking oils, and salad oils, while seed meal from which the oil has been extracted, having a high protein content, is generally processed into animal feeds. About two percent of this meal is further refined for human consumption.
In the recovery of seed oils, the industry consumes large quantities of extraction solvent. Hexane has long been recognized as the standard solvent in the industry, due in part to its low cost relative to other solvents and in part to its physical and chemical properties. However, in recent years incentive has developed for the replacement of hexane as the solvent of choice for oilseed extraction. Increasing hexane costs and possibilities of supply shortages account for some of this incentive. In addition, for health and safety reasons, solvent specifications in the industry, precautions to prevent exposure of workers to hexane, and relevant hydrocarbon emission standards may be tightened. Furthermore, interest in producing an upgraded seed meal, for instance a meal suitable for human consumption without the need for secondary extraction or other refining, has given rise to attempts at use of alternate extraction solvents.
Of particular relevance to the present invention is the prior art relating to extraction of oilseeds with an isopropanol-based solvent. Characteristics of isopropanol extraction are generally well known and are described, for instance, by Harris et al in a three-part publication entitled "Isopropanol as a Solvent for Extraction of Cottonseed Oil" (J. Am. Oil Chem. Soc., November 1947, Vol. 24, p. 370-375; December 1949, Vol. 26, p. 719-723; and July 1950, Vol. 27, p. 273-275). More recent developments in oilseed processing with isopropanol are outlined by Youn and Wilpers in U.S. Pat. No. 4,298,540.
An important consideration in oilseed extraction with an isopropanol-based solvent is control over the water content of the solvent. Water, which enters the extraction process, for example, with the seed feedstock, in live steam employed either as a heat source or as an aid to one or more process separations, in solvent make-up, etc., often tends to build-up in the process, specifically in one or more process solvent recycle streams. It is well known (see, for instance, the publication of R. K. Rao and L. K. Arnold in J. Am. Oil Chem. Soc., August 1957, Vol. 34, pp. 401-404) that increasing the water content of an isopropanol-based extraction solvent diminishes the solubility of seedoils in the solvent at a given temperature and decreases the oil recovery efficiency of the overall extraction process. As a result, some method for controlling water content of solvent in the extraction process is necessary.
In the common conventional oilseed extraction process employing a hexane extractant, the relative insolubility of water in hexane permitted control of water in the process by simple phase separation. Because of the miscibility of isopropanol and water, phase separation cannot be applied in a like manner in the isopropanol solvent extraction process.
The principal approach which has been taken in the oilseed processing art to the control of water content in a process utilizing an isopropanol-based solvent involves restrictions upon the quantity of water introduced into the system, e.g., in the feedstock and in process steam flows. Water entering the system is limited to that which naturally exits the process in oil and meal products when the process is being practiced at a desired level of water content. This approach, however, is difficult to implement and control. It would be possible to reduce process water content by distillation of a liquid solvent stream, for instance, in a manner such as that described in U.S. Pat. No. 4,298,540. This, however, entails a substantial and costly addition to process energy and equipment requirements.